The prior art has disclosed hydrostatic transmissions for mobile working machines in which a hydrostatic pump (primary unit) and a hydrostatic motor (secondary unit) are fluidically connected to one another by means of a closed circuit. An internal combustion engine, for example a diesel engine of the mobile working machine, is coupled rotationally conjointly to the primary unit, and an output, for example an axle or a wheel of the mobile working machine, is coupled rotationally conjointly to the secondary unit. The mobile working machine thus has a traction drive which has a hydrostatic transmission.
The document EP 1 960 699 B1 discloses a hydrostatic transmission, with which braking can also be performed. Here, in relation to traction operation, the power flows in the reverse direction from the output, via the secondary unit acting as a pump and via the primary unit acting as a motor, to the internal combustion engine, which is then driven in a passive cranking operating mode. The highly pressurized working line of the closed circuit is in this case safeguarded by means of a pressure-limiting valve, by means of which, too, a part of the braking power can be dissipated during the braking operation. In the case of said solution, however, the braking operation is triggered only when the driver explicitly expresses the demand by means of an actuation of the brake pedal. The driver must therefore also actuate the brake pedal in order to avoid excessively high rotational speeds of the internal combustion engine.
The documents DE 10 2014 211 393 A1 and US 2014/0372000 A1 each disclose a hydrostatic transmission, wherein it is sought to protect the internal combustion engine against overspeeding. For this purpose, a speed control system is described which identifies an overshooting of the braking power of the internal combustion engine and thereupon automatically triggers a high-power braking operation. A part of the braking power is output to the internal combustion engine, whereas another part of the braking power is converted by means of the pressure-limiting valve of the high-pressure line in question into heat. Here, a combination of the actual rotational speed of the internal combustion engine and a further signal, such as a rotational speed deviation and a speed deviation, are used. A disadvantage of this hydrostatic transmission is that situations arise in which it is not sufficient to protect the internal combustion engine on the basis of the described criteria. Furthermore, the transmission does not take into consideration the large differences that can exist between the various mobile working machines in question.
Furthermore, from the prior art, a hydrostatic transmission is known which, in the event of an overshooting of a rotational speed threshold or of an activation threshold of the rotational speed of the internal combustion engine, triggers a high-power braking operation by virtue of the primary unit being pivoted back. Thus, although the internal combustion engine is protected, its possible braking power is not optimally utilized.
By contrast, it is the object of the disclosure to provide a hydrostatic transmission in the case of which, during a braking operation, the internal combustion engine is reliably protected against overspeeding and at the same time discharges or dissipates the maximum possible braking power.
Said object is achieved by means of a hydrostatic transmission having the features disclosed herein.